Somebody is clicking them. Google Revenue for last year was $28 Billion. 96% from search advertising.
Why not click on an ad if they are offering what you are searching for?
I do it all the time because I know that when I want to find something to buy they will be relevant to my search. This is different from Ads appearing in the content network on Hubpages.
Unfortunately the relevancy of Adsense displayed on Hubpages bottomed out when they made all the changes. Might have a lot to do with subdirectory changes.
I see a lot of totally irrelevant Adsenses Ads... but the Ads displayed on Google itself are usually very relevant.